After Effects - Mask with rotobrush

Hello everyone, 

I am very new in AE. I am having difficulty with rotobrush and masking. 

I have 2 videos:
1. a man walking: I used this one with rotobrush in order to have the walking silhouette. 
2. an "abstract" video: I want to use the walking man "cut-out" (using rotobrush) as a mask to this one. So I would be seeing an abstract video with  the shape of the walking man. 

It may sound very basic but I couldn't find on YouTube either. Maybe I am using writing keywords. 

Thank you